H,Cl和F原子钝化Cu_2ZnSnS_4(112)表面态的第一性原理计算

采用基于密度泛函理论的第一性原理计算方法系统研究了Cu_2ZnSnS_4体相的晶格结构、能带、态密度及表面重构与H,Cl和F原子在Cu_2ZnSnS_4(112)表面上的吸附和钝化机理.计算结果表明:表面重构出现在以金属原子Cu-Zn-Sn终止的Cu_2ZnSnS_4(112)表面上,并且表面重构使表面发生自钝化;当单个H,Cl或F原子吸附在S原子终止的Cu_2ZnSnS_4(112)表面上时,相比于桥位(bridge)、六方密排(hcp)位和面心立方(fcc)位点,三种原子均在特定的顶位(top)吸附位点表现出最佳稳定性.当覆盖度为0.5 ML时,无论H,Cl还是F原子占据Cu_2ZnSnS_4(112)表面的2个顶位均具有最低的吸附能.以S原子终止的Cu_2ZnSnS_4(112)表面在费米能级附近的电子态主要由价带顶部Cu-3d轨道和S-3p轨道电子贡献,此即表面态.当H,Cl或F原子在表面的覆盖度达0.5 ML时,费米能级附近的表面态降低,其中H原子钝化表面态的效果最佳,Cl原子的效果次之,F原子的效果最差.表面态降低的主要原因在于吸附原子从S原子获得电子致使表面Cu原子和S原子在费米能级处的态密度峰几乎完全消失.     

纳米线高应变率拉伸超塑性

应用分子动力学方法,采用嵌入势EAM与Buckingham势,对金属Cu、半导体化合物CuInSe2和陶瓷化合物MgO纳米线进行拉伸模拟,考察其拉伸应力-应变曲线,并分析拉伸过程中的结构变化.发现当以高于临界应变率的速率对纳米线进行拉伸时,纳米线由脆性断裂向韧性断裂转变,且其延伸率可以超过100%,表现出超塑性的特性,而以较低应变率拉伸时,纳米线仍然表现为脆性断裂,这表明纳米线材料的超塑性对于应变率高度敏感.通过观察纳米线在拉伸过程中的结构变化,发现高应变率拉伸时由于CuInSe2与Cu纳米线晶体结构发生非晶化,在这一转变过程中大量能量被吸收,因而导致其塑性变好.而MgO纳米线则发生面心立方结构向环形结构的相变,相变的发生同样导致了能量的吸收,从而使其塑性大大改善.     

LaMnO3/SrMnO3超晶格应变下的电磁性质

采用基于第一性原理的密度泛函理论研究正交超晶格结构LaMnO3/SrMnO3 (LMO/SMO)晶格结构、轴向应变及电磁结构的影响.理论计算结果表明,LMO/SMO随a、b、c及ac方向的轴向应变变化均是铁磁性状态比反铁磁性状态更稳定.Sr掺杂后,LMO正交结构仍保留明显的晶格畸变,SMO的面心立方结构的晶格畸变增加,Mn的3d态及O的2p态在不同的轴向应变下更加复杂.Mn的3d态轨道分波态密度随着负应变的增大,自旋向上和自旋向下态密度趋向于对称,磁性趋向于零.     

氧在La/SrMnO3（001）表面吸附、解离和表面扩散的第一性原理

采用基于密度泛函的第一性原理的方法,同时结合Nudged Elastic Band方法,系统研究O原子和O2分子在La/SrMnO3(001)表面的吸附过程。给出O2分子在MnO2、LaO和SrO外表面的解离路径和势垒;研究O原子在三种表面的扩散过程,计算出各扩散路径的势垒和最稳定的吸附位置的基本情况。在此基础上,通过比较解离、扩散和放氧环节的激活能数据,提出O2的解离和表面放氧过程均为速率控制步骤。进一步为La/SrMnO3作为固体阴极材料时决定性能的第一步吸附反应提供理论数据的支持.     

Lattice structures and electronic properties of WZ-CuInS_2/WZ-CdS interface from first-principles calculations

Using the first-principles plane-wave calculations within density functional theory, the perfect bi-layer and monolayer terminated WZ-CIS(100)/WZ-Cd S(100) interfaces are investigated. After relaxation the atomic positions and the bond lengths change slightly on the two interfaces. The WZ-CIS/WZ-Cd S interfaces can exist stably, when the interface bonding energies are-0.481 J/m~2(bi-layer terminated interface) and-0.677 J/m~2(monolayer terminated interface). Via analysis of the density of states, difference charge density and Bader charges, no interface state is found near the Fermi level.The stronger adhesion of the monolayer terminated interface is attributed to more electron transformations and orbital hybridizations, promoting stable interfacial bonds between atoms than those on a bi-layer terminated interface.     

硝酸剥蚀对纳米铜粉在润滑油中分散特性的影响

对纳米铜粉进行硝酸剥蚀和表面改性,研究剥蚀程度和表面改性剂的用量对纳米铜粉分散性能的影响.应用X射线衍射(XRD)、透射电镜(TEM)和傅里叶红外光谱(FI-IR)对剥蚀和改性后纳米铜粉的组成、颗粒大小、表面状况进行表征.结果表明:硝酸剥蚀可以起到细化纳米铜粉颗粒的作用,聚乙烯吡咯烷酮(PVP)在对纳米铜粉表面改性的同时抑制Cu2O的产生.剥蚀改性后的纳米铜粉的分散稳定性明显优于未剥蚀改性的纳米铜粉.     

氟化度及单轴应力对氟化石墨烯结构稳定性和能带的影响

采用基于密度泛函数理论的第一性原理,通过计算研究了四种(Chair、Zigzag、Boat、Armchair)双面全氟化石墨烯(N_C∶N_F=1∶1)构型,发现Chair型氟化后的石墨烯构型最稳定。在此基础上,系统研究了以Chair形式氟化时不同氟化度和单轴应力对氟化石墨烯结构稳定性与能带的影响.计算结果表明:氟化度越高氟化石墨烯结构越稳定,且双面氟化石墨烯较单面氟化更稳定。对构造的全氟化石墨烯体系沿X方向施加压(拉)应变时,石墨烯体系的起伏高度随单轴应变的增加而减小,导带底和价带顶均发生微小移动、带隙逐渐减小,带隙在轴向压应力作用下减小得更快.     

Lattice structures and electronic properties of CIGS/CdS interface:First-principles calculations

Using first-principles calculations within density functional theory, we study the atomic structures and electronic properties of the perfect and defective （2VCu＋ Incu） CulnGaSe2/CdS interfaces theoretically, especially the interface states. We find that the local lattice structure of （2VCu＋ InCu） interface is somewhat disorganized. By analyzing the local density of states projected on several atomic layers of the two interfaces models, we find that for the （2VCu＋InCu） interface the interface states near the Fermi level in CulnGaSe2 and CdS band gap regions are mainly composed of interracial Se-4p, Cu-3d and S-3p orbitals, while for the perfect interface there are no clear interface states in the CulnGaSe2 region but only some interface states which are mainly composed of S-3p orbitals in the valance band of CdS region.     

Thermal properties of high-k Hf1-xSixO2

Classical atomistic simulations based on the lattice dynalnics theory and the Born core-shell model are performed to systematically study the crystal structure and thermal properties of high-k Hfl-xSixO2. The coefficients of thermal expansion, specific heat, Griineisen parameters, phonon densities of states and Debye temperatures are calculated at different temperatures and for different Si-doping concentrations. With the increase of the Si-doping concentration, the lattice constant decreases. At the same time, both the coefficient of thermal expansion and the specific heat at a constant volume of Hf1-mSixO2 also decreases. The Griineisen parameter is about 0.95 at temperatures less than 100 K. Compared with Si-doped HfO2, pure HfO2 has a higher Debye temperature when the temperature is less than 25 K, while it has lower Debye temperature when the temperature is higher than 50 K. Some simulation results fit well with the experimental data. We expect that our results will be helpful for understanding the local lattice structure and thermal properties of Hf1-mSixO2.